1. Field:
The subject invention is in the field of methods and apparatus for attenuating the noise produced by jet engines. More specifically it is in the field of apparatus which is adjustable and provides two modes of operation, non-attenuated, when the noise produced causes no problems and attenuated. It is generally known in the art that attenuation reduces the overall propulsion efficiency and that the better solutions to the problems of attenuation of jet engine noise provide the better compromises between noise attenuation and the efficiency given up to achieve the attenuation. Still more specifically, the invention is in the field of apparatus for intermixing the ambient air in which an engine is operated and effluent gases.
2. Prior Art:
The problem of attenuating the noise of jet engines is a long standing one and it has become more pressing and difficult as a result of increasingly stringent legal limitations on the noise produced by airplanes. The limitations are such that owners of airplanes equipped with pure jets face the choice of re-engining the airplanes, if feasible, retiring the airplanes or finding ways of attenuating the noise without unacceptable reduction in operating efficiency.
U.S. Pat. No. 4,422,524 covers a nozzle for a jet which provides a cruise mode configuration and a sound-suppression mode. The apparatus of this patent involves a relatively large number of individual moving parts, all of which must move relative to fixed surfaces with seals between all the moving and fixed parts. It will be recognized by those skilled in the art that the number of moving parts and the sealing requirements present design, manufacturing, operational and maintenance problems which will work against the overall utility of the patented solution to the described problem. These problems are the result in large part of the severe operating conditions in the vicinity of the jet exhausts. The patent provides no data related to the effectiveness of the patented apparatus in regard to sound suppression or attenuation or to thrust losses associated with the attenuation obtained.
Another solution is shown in U.S. Pat. No. 3,550,721. The basic approach for noise suppression is to introduce ambient air into the exit flow and increase the exit area to accommodate the increased flow. This approach is well known in the art. The embodiment in this instance involves a multiplicity of complex parts pivoted to fixed structure along with longitudinally movable elements for uncovering inflow areas for the ambient air and for increasing the nozzle exit area. In this instance also it will be understood that the number and complexity of moving parts in the jet exhaust environment will present problems working against the utility of the invention. In this instance also there is no data presented related to the effectiveness of the invention in terms of suppression achieved relative to thrust lost in achieving it.
U.S. Pat. No. 3,592,291 shows a method and apparatus for suppressing jet noise and thrust augmentation is cited. There is no provision for geometric changes to suit the conditions which vary between cruise and sound suppression operations. The method involves mixing ambient flow into the jet gases. There are no moving parts. However, it is now well known in the art that sound suppression with thrust augmentation has not yet been achieved and that some form of variable nozzle geometry is needed to achieve useful compromises between noise suppression and thrust lost in obtaining it.
There is profuse prior art also for sound suppression of turbofan (fan jet) engines. With fan jet engines the methods involve mixing primary flow, fan flow and/or ambient air flow and in each case variable geometry embodiments are required. Examples of this art are found in U.S. Pat. Nos. 3,568,794, 3,579,993, 4,077,206 and 4,117,671. In the embodiments in these patents flows are mixed and ambient flow is inducted but in no case is primary flow vented outboard through the nozzle wall into the ambient flow.
It has been determined that there are advantages to be gained with fan jet engines by varying the bypass ratio. The bypass ratio, briefly, is the ratio of the mass of the fan flow to the mass of the primary flow. Ratios in the range of 2/1 to 5/1 are common. A mechanism for varying the bypass ratio is disclosed in U.S. Pat. No. 3,779,282 by Klees. This mechanism is relevant prior art to the apparatus of the subject invention and will be referred to in the Detailed Description of the Invention.
It can be understood from the above discussion that there is a longstanding need for apparatus which enables achieving an improved compromise between the suppression of the noise of a jet engine and the costs required to achieve the suppression, the costs including performance degradation and manufacturing and maintenance costs. In particular there is a need for such apparatus which can readily be applied to existing engines, as different from apparatus which must be designed and built into the engine.
It is a prime objective of the subject invention to provide such apparatus. Inherent further objectives are that the apparatus have few moving parts and be simple to manufacture, operate and maintain.